Behm JE, Ives AR &
Boughman JW. (2010) Ecological disturbance and the collapse of a species pair
through hybridization. American Naturalist.

Species that evolved through
ecological speciation and lack intrinsic genetic incompatibilities may
nonetheless be maintained by extrinsic postmating isolating barriers that
impose selection against hybrids. These species, however, may be vulnerable to
a breakdown in postmating isolation. Here, we investigate a model system for
ecological speciation: sympatric limnetic-benthic pairs of threespine
sticklebacks. Recently, stickleback hybrid abundance in Enos Lake has
increased. Given that ecological selection against hybrids was historically an
important component of total reproductive isolation, we tested whether
ecologically dependent postmating isolation is still functioning. We compared
body shape, diet, growth, and survival in present day Enos fish to the
undisturbed Paxton Lake species pair, and to historical Enos Lake data. In both
Paxton and historical Enos data, we found a strong correlation between body
shape and diet; however, in present-day Enos fish this correlation was absent.
Using fitness estimates based on growth rates and survival, we found no
evidence of selection against intermediate morphologies. It appears that
postmating isolation has broken down, allowing hybrids to persist and
contributing to the collapse of the species pair.